Corrosion Estimation Device and Method

This application is a national phase entry of PCT Application No. PCT/JP2022/020487, filed on May 17, 2022, which application is hereby incorporated herein by reference.

The present invention relates to a corrosion estimation device and a corrosion estimation method for estimating corrosion of a metal member buried in soil.

Social infrastructure facilities that support our life have been rapidly developed since the high economic growth period. For this reason, it is said that in 2030, facilities that are 50 years old after construction will occupy more than half of the entire facilities. To prevent failure of these aging infrastructure facilities, maintenance operation by periodic inspection has been conventionally performed. However, in recent years, inspection work has been delayed due to an increase in aging facilities and a decrease in inspection technicians, and appropriate measures cannot be taken for objects subject to deterioration to be inspected, which may cause a serious accident such as collapse. Further, since visual inspection is difficult depending on an installation place of the facility, the inspection itself is not performed in many cases. A typical example of the place where the visual inspection is difficult is in soil.

Therefore, in recent years, research has been actively conducted for establishing a technique of predicting and estimating a deterioration state of a facility buried in soil. If this prediction and estimation technique is established, it becomes possible to distinguish between a subject with severe deterioration and a subject with slow deterioration without performing a site inspection, and not only safety is secured by preferentially updating a subject with fast deterioration progression, but also efficiency in terms of cost is expected to be improved by using a subject with slow deterioration progression for a longer time. Since the main cause of deterioration of a buried steel material is soil corrosion, it is important to grasp the relationship between a dominant factor of soil corrosion and a corrosion rate in order to predict and estimate the deterioration state.

However, soil is a special environment in which three phases of a solid phase, a gas phase, and a liquid phase coexist, and it is considered that there are various factors contributing to a corrosion reaction (Non Patent Literature 1). In particular, solid phase information specific to soil can be important information for understanding soil corrosion. Examples of this unique information include water that determines a presence or absence of occurrence of the corrosion reaction, and a soil particle size distribution as solid phase information that affects a state of oxygen.

A soil particle gap structure and a particle packing density change depending on a difference in the particle size distribution, which greatly affects ease of oxygen supply from a soil surface layer and a wetted area of a surface of the metal member by water captured by capillary action. In addition, a pressure of the water captured by capillary action is also different due to a smallness of the soil particle gap, and under a condition that the water is captured with a stronger pressure, a thin water film is formed on the surface of the metal member, and an environment in which soil corrosion easily proceeds is generated. Therefore, it is important to utilize the particle size distribution of soil in order to predict and estimate soil corrosion.

Regarding the particle size distribution, useful information can be read from a frequency distribution curve represented by a horizontal axis particle size and a vertical axis frequency. For example, the size of the particle size distribution can be evaluated by a shape of the curve, and in a case where the shape of the frequency distribution curve is sharp, it means that a large amount of soil having the same particle size is included because the frequency of a certain particle size is high, and it can be seen that the distribution of the particle size is narrow. Conversely, when the shape of the frequency distribution curve is broad, it can be seen that soil of any particle size is included, and the distribution of the particle size is wide. A porosity of the particles varies depending on the size of the particle size distribution, and thus not only affects a rate of drainage, but also affects the formation of the thin water film because a capturing power of water by capillary force based on a soil particle gap size varies.

Non Patent Literature 1: Y. Wan et al., “Corrosion Behaviors of Q235 Steel in Indoor Soil”, International Journal of ELECTROCHEMICAL SCIENCE, vol. 8, pp. 12531-12542, 2013.

As described above, useful information can be obtained from a particle size distribution, but it is troublesome to convert a result of actually measuring the particle size distribution into a soil particle packing density and a capturing power of water by capillary force. Even if the particle size distribution can be grasped, it cannot be said that the soil particle packing and a wettability of a surface of a metal member in an actual burying environment can be accurately evaluated. As described above, conventionally, there has been a problem that accurate estimation of corrosion of a metal member buried in the ground is not easy.

Embodiments of the present invention has been made to solve the above problems, and an object thereof is to enable more accurate and easier estimation of the corrosion of the metal member buried in the ground.

A corrosion estimation device according to embodiments of the present invention includes a soil conditioner that performs a pretreatment for measuring a moisture characteristic of soil, a measurement instrument that measures the moisture characteristic of the soil pretreated by the soil conditioner, and a calculator that estimates corrosion of a metal member buried in the soil from the moisture characteristic measured by the measurement instrument.

In addition, a corrosion estimation method according to embodiments of the present invention includes a first step of performing the pretreatment for measuring the moisture characteristic of the soil, a second step of measuring the moisture characteristic of the soil subjected to the pretreatment, and a third step of estimating the corrosion of the metal member buried in the soil from the measured moisture characteristic.

As described above, according to embodiments of the present invention, since the corrosion of the metal member is estimated from the moisture characteristic of the soil in which the metal member is buried, the corrosion of a metal member buried in the ground can be more accurately and easily estimated.

Hereinafter, a corrosion estimation method according to an embodiment of the present invention will be described with reference to. In this method, first, in a first step S, a pretreatment for measuring a moisture characteristic of soil is performed. The pretreatment can be removal of moisture from the soil of interest. For example, in a case where soil collected from a place where a metal member (for example, a steel material) to be estimated for corrosion is buried is wet soil, a soil particle mass may be formed by water trapped in a soil particle gap. When the moisture characteristic is measured in the presence of the soil particle mass, an original moisture characteristic may not be evaluated because a particle size distribution is different. In such a case, as the pretreatment, it is important to remove the water that is a cause of the soil mass.

Next, in a second step S, the moisture characteristic of the soil subjected to the pretreatment is measured. The moisture characteristic is a water content of the soil. In addition, the moisture characteristic is a matrix potential value at a specified soil water content.

Next, in a third step S, the corrosion of the metal member buried in the soil is estimated from the measured moisture characteristic.

Next, a corrosion estimation device for performing the above-described corrosion estimation method will be described with reference to. The corrosion estimation device includes a soil conditionerthat performs the pretreatment for measuring the moisture characteristic of the soil, a measurement instrumentthat measures the moisture characteristic of the soil pretreated by the soil conditioner, and a calculatorthat estimates the corrosion of the metal member buried in the soil from the moisture characteristic measured by the measurement instrument. The measurement instrumentmeasures the water content of the soil. In addition, the measurement instrumentmeasures a matrix potential at the specified soil water content.

As illustrated in, the soil conditionerincludes a soil accommodating unit, a drying unit, and a stirring unit.

The soil accommodating unitstores a soil sample collected from the place where the metal member for which soil corrosion is to be estimated is buried. An amount of soil to be stored in the soil accommodating unitmay be any amount as long as the moisture characteristic measurement can be performed, and for example, it is preferable to prepare about 500 mL. A shape of the soil accommodating unitis not limited as long as it is a storage container having a size capable of storing the above-described amount of soil. A material of the storage container constituting the soil accommodating unitcan be arbitrarily determined by a user.

In a case where the storage container constituting the soil accommodating unitis metal, when the wet soil is stored, a corrosion reaction with the wet soil may occur depending on the type of metal, the storage container may be deteriorated, and a corrosion product may be mixed into the wet soil, and the original soil characteristic may be impaired. Therefore, it is preferable to avoid metal when selecting the material of the storage container constituting the soil accommodating unit.

In addition, when a method of applying heat is selected when the soil stored in the soil accommodating unitis dried by the drying unit, it is preferable to avoid a material weak to heat. For example, a heat-resistant polymer resin or glass can be adopted as the material constituting the soil accommodating unit.

In a case where the soil stored in the soil accommodating unitis wet soil, the soil particle mass may be formed by the water trapped in the soil particle gap. When the moisture characteristic is measured in the presence of the soil particle mass, the original moisture characteristic may not be evaluated because the particle size distribution is different. In order to remove the water that is the cause of the soil mass, the drying operation of the soil in the soil accommodating unitis performed in the drying unit. As the drying method in the drying unit, for example, heat can be applied to increase a temperature of the soil accommodating unitand dry the soil. Further, the inside of the soil accommodating unitcan be decompressed and vacuum-dried.

However, in the case of adopting the method of drying by applying heat, it is necessary to select a material that the soil accommodating unitcan endure up to a temperature set by the user. In addition, in a case where the soil in the soil accommodating unitcontains humus and exhibits a black color, there is a possibility that chemical components derived from organic substances are modified by heat and properties inherent in the black soil are lost. Therefore, the heating temperature as the pretreatment is preferably kept at an upper limit of 50° C. or lower.

In addition, when the decompression method is adopted, the storage container used for the soil accommodating unitneeds to be a material that can withstand decompression. Therefore, when the decompression method is used, the material of the storage container is preferably glass.

The drying operation by the drying unitis ended when the soil water content in the soil accommodating unitreaches 0%. For example, by installing a water content sensor in the soil accommodating unit, it is possible to measure the water content of the soil stored in the soil accommodating unit. The method for drying the soil in the drying unitis not limited to the methods described above as long as the soil water content in the soil accommodating unitcan be set to 0%.

The stirring unitperforms stirring for unraveling the soil particle mass in the soil in the soil accommodating unitin which the soil water content has reached 0% by the drying by the drying unit. The stirring of the soil in the stirring unitis not limited as long as it is a method in which all the soil particle masses are eliminated. For example, stirring can be performed by the stirring unitby stirring two rod-shaped objects in a circle. In addition, a mechanism similar to an automatic stirrer adopted in a food factory or the like can be adopted as the stirring unit.

As illustrated in, the measurement instrumentincludes a soil accommodating unit, a soil saturation unit, a water content change measurement unit, a matrix potential measurement unit, and a maximum corrosion rate measurement unit.

The measurement instrumentmeasures the moisture characteristic of the soil subjected to the pretreatment step by the soil conditioner. First, the pretreated soil is transferred from the soil accommodating unitof the soil conditionerto the soil accommodating unit. An accommodating container used for the soil accommodating unitis not particularly limited in shape, material, and the like as long as the accommodating container has a capacity capable of storing all the pretreated soil in the soil accommodating unitand is capable of measuring the moisture characteristic of the soil. In addition, in a case where an electrochemical measurement in the maximum corrosion rate measurement unitis performed, it is necessary to bury an electrode (for example, a metal electrode) when the soil is transferred from the soil accommodating unitto the soil accommodating unit.

The soil saturation unitperforms a treatment of removing a gas phase of the soil particle gap from the soil stored in the soil accommodating unitand entirely filling the soil with a liquid phase, that is, saturating the soil. Examples of the saturation treatment include a water immersion deaeration method and a water absorption deaeration method. The water immersion deaeration method is a method in which the soil accommodating unitis placed in a water immersion decompression container filled with water, and the inside of the water immersion decompression container is gradually decompressed by a vacuum pump or the like to be saturated. When the generation of air bubbles from the soil accommodating unitis no longer confirmed, that is, after the gas phase in the soil is entirely filled with the liquid phase, a pressure in the container is gradually returned to an atmospheric pressure to complete the treatment.

In the water absorption deaeration method, the soil accommodating unitis connected to a decompression water supply device, and decompression of the inside of the soil accommodating unitand water supply from a water supply bottle to the soil accommodating unitare alternately performed by a vacuum pump or the like. The treatment is completed by repeating the above-described decompression and water supply until no air bubbles come out of an aspirator bottle constituting the vacuum pump. There is no limitation as long as the method can saturate the pretreated soil in the soil accommodating unit.

The water content change measurement unitmeasures a water content change of the soil subjected to the soil saturation treatment in the soil accommodating unit. The soil in the soil accommodating unithas the maximum soil water content by the saturation treatment of the soil saturation unit. The water content sensor is inserted into the saturated soil, and a temporal change of the soil water content is measured. In the measurement of the temporal change of the soil water content, a filtration filter is installed on a bottom surface of the soil accommodating unitin order to reproduce drainage of an actual environment. A pore size of the filtration filter is not limited, but if the pore size is too fine, a rate at which water escapes from the filter is slow, and a rate of drainage may be underestimated. Therefore, the pore size of the filter is preferably about 50 μm.

In the measurement by the water content change measurement unit, the soil accommodating unitis held at a constant temperature and a constant humidity environment. As this environment, for example, the annual average temperature and the annual average humidity in the Kanto region can be adopted. The saturated soil is allowed to stand under the constant temperature and the constant humidity environment, the soil water content is gradually reduced by natural drainage, and the temporal change of the water content is measured by the water content sensor.

illustrates an example of a graph representing the temporal change of the soil water content in the soil accommodating unit.illustrates three types of sand, silt, and clay, and illustrates an example in which the sand, silt, and clay contain larger soil particles in that order, so that the water content decreases earlier in that order.

The matrix potential measurement unitmeasures a relationship between a matrix potential representing how much the water in the soil is held by a strong force with respect to the soil in the soil accommodating unitsaturated by the treatment of the soil saturation unitand the soil water content. When a maximum corrosion rate described later is measured, the metal electrode is buried in advance before the measurement is performed.

illustrates an example of a soil moisture characteristic curve representing the relationship between the matrix potential and the soil water content. The unit of the matrix potential is Pa, which indicates that water is held with a strong force when a value is small, that is, when an absolute value is large.illustrates the three types of sand, silt, and clay. When the matrix potential is a value close to zero, the soil is saturated, and mainly water is retained by the surface tension of the soil particle gap. As the soil water content decreases, the matrix potential becomes smaller, and water is strongly adsorbed to a small soil particle gap.

In, in the sand, water is mainly held by capillary force, and most of the water is drained at a high matrix potential. On the other hand, in the clay, water is retained by intermolecular force and force due to osmotic pressure in addition to capillary force, so that it can be seen that the water does not escape to a smaller matrix potential. For the measurement of the matrix potential and the creation of the moisture characteristic curve, for example, a moisture characteristic curve/unsaturated hydraulic conductivity coefficient measurement device HYPROP 2 and a matric potential measuring device WP4C manufactured by METER can be used. As long as the matrix potential can be measured and the moisture characteristic curve can be created, the device is not limited to the devices described above.

The maximum corrosion rate measurement unitperforms electrochemical measurement for calculating the maximum corrosion rate of the soil. The electrochemical measurement is performed by using the metal electrode buried in the soil in advance. Details of a method of calculating the maximum corrosion rate or the like will be described later. A relationship between the maximum corrosion rate obtained by the maximum corrosion rate measurement unitand the matrix potential obtained by the matrix potential measurement unitis derived. The electrochemical measurement is not limited as long as a charge transfer resistance Rcan be measured, such as with an impedance method or a direct current polarization resistance method. A conversion coefficient K for calculating a corrosion current density from the measured charge transfer resistance value is calculated by comparing with a weight change after the test, and the maximum corrosion rate is derived from the following Expressions (1) and (2).

In the Expressions (1) and (2), irepresents a corrosion current density [μA/cm], K represents a conversion coefficient, Rrepresents a charge transfer resistance [Ω·cm], r represents a maximum corrosion rate [mm/year], z represents an ionic valence, ρ represents a density [g/cm], F represents a Faraday constant [C], and M represents an atomic weight [mol].

As will be described later, a process in the maximum corrosion rate measurement unitmay not be performed again after the relationship between the maximum corrosion rate and the matrix potential is derived. If the relationship is known from the beginning, the process in the maximum corrosion rate measurement unitdoes not need to be performed.

The calculatorestimates the soil corrosion of the buried steel material based on a result obtained by the measurement instrument.

Results measured by the water content change measurement unitand the matrix potential measurement unitin the measurement instrumentare stored in a memory of the calculator.

The calculatorestimates the soil corrosion based on each measurement result saved (stored) in the memory. The progress of the soil corrosion reaction is determined by a wetted area of a surface of the buried metal member and an oxygen partial pressure. The wetted area depends on a potential of the water trapped in the soil particle gap of the soil. In the oxygen partial pressure, after the soil particle gap is filled with water such as rain, the water permeates and diffuses deep into the ground as gravitational water, and oxygen diffuses from a surface layer to the ground to supply oxygen to the surface of the metal member.

The oxygen can be dissolved in water and reach the surface of the metal member as dissolved oxygen, but since a diffusion rate of the dissolved oxygen is 10times slower than the diffusion rate of gaseous oxygen, the oxygen required for the corrosion reaction is more likely to be supplied, as a distance of diffusion in the soil as a gas is longer. Drainage in the soil progresses, and the water (liquid phase) in the soil particle gap is replaced by air (gas phase), so that the distance in which gaseous oxygen can be diffused becomes long.

Since there is a saturated state immediately after the start of the test, corrosion proceeds by a mechanism similar to that of aqueous solution corrosion, and a water film exceeding a limiting diffusion layer is formed, and the diffusion rate of the dissolved oxygen is rate-determined, indicating a constant corrosion rate. When the soil water content decreases and the liquid phase in the soil is replaced by the gas phase, oxygen is efficiently supplied while the wetted area of the surface of the metal member is secured, so that the corrosion rate rapidly increases and reaches the maximum corrosion rate. A maximum value of the corrosion rate should be higher as the water film formed on the surface of the metal member is thinner, and it is necessary for the wetted area of the surface of the metal member to be secured. As time further elapses and the soil water content decreases, the water film becomes thin, but the wetted area of the surface of the metal member cannot be secured, and the corrosion rate decreases.

Next, an output of the estimation result in the calculatorwill be described. First, since a constant corrosion rate is maintained after the start of the test, the value is arbitrarily determined. If it is desired to determine a more accurate initial corrosion rate, an electrical resistance value of the saturated soil is measured, and a larger initial corrosion rate can be set if the electrical resistance value is small, and a smaller initial corrosion rate can be set if the value is large. Subsequently, a time at which the corrosion rate rapidly increases is set. In a water content change curve shown in, it is assumed that the corrosion rate rapidly increases at a certain constant soil water content, and a time when a soil water content θpeak is reached is recorded. In the example of, the sand is recorded as time t, the silt as time t, and the clay as time t.

θpeak can be arbitrarily determined by the user, but θpeak is preferably set within a range of 10 to 30%, since the corrosion rate increases in a situation where the soil is dried from a saturated state and the liquid phase and the gas phase are present in a well-balanced manner. Since the rapid increase in the corrosion rate is often observed when the soil water content is 20%, θpeak=20% can be set.

Subsequently, in the moisture characteristic curve illustrated in, the matrix potential value at θpeak is recorded. In, the sand is recorded as a matrix potential value ψ, the silt is recorded as a matrix potential value ψ, and the clay is recorded as a matrix potential value ψ.

Subsequently, the maximum corrosion rate is determined. When soil having the same water content is compared, in the soil having a high matrix potential, water is held only in a region where the soil particle gap is locally narrowed, and similarly, the wetting of the surface of the metal member locally forms a thick water film. As a result, the maximum corrosion rate in the soil having the high matrix potential becomes smaller.

On the other hand, in the soil having a low matrix potential, a narrow region of the soil particle gap is uniformly formed over the entire soil, and water is strongly retained, so that a water film that is thin and in which the wetting of the surface of the metal member is uniform is formed. This increases the maximum corrosion rate in the soil with the low matrix potential.